Various types of biometric systems are used more and more in order to provide for increased security and/or enhanced user convenience. In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance, and user acceptance.
Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues.
All capacitive fingerprint sensors provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor.
Since a capacitive sensor detects a finger based on the capacitance between the finger and the sensor, the distance between the sensing surface and the sensing structures directly influence the contrast and the resolution of the fingerprint image captured by the measurement. This traditionally did not pose a problem as the thickness of the cover material could be chosen with little design pressure. However, according to new design trends it is desirable to place the sensor under thick cover glass and to eventually integrate the fingerprint sensor within a display arrangement.
This presents a challenging problem. The source of this problem is not only related to weakening of the capacitive signal by increased finger-to-sensor distance. Commercially available capacitive touch sensors may function well through thick cover glasses. However, a problem is related to the loss of resolution and image contrast as the finger-to-sensor distance is increased. This is caused by the fact that distinguishing minute capacitance variations due to finger corrugations from a large background “average” that comes from the sum of all the ridges and valleys “visible” to a pixel becomes extremely difficult at large finger-to-sensor distances.
Accordingly, it is desirable to provide a fingerprint sensor overcoming some of the above described difficulties associated with capacitive sensing through thick cover layers.